This function returns the process group ID of the foreground process
group associated with the terminal open on descriptor filedes.

If there is no foreground process group, the return value is a number
greater than 1 that does not match the process group ID of any
existing process group. This can happen if all of the processes in the
job that was formerly the foreground job have terminated, and no other
job has yet been moved into the foreground.

In case of an error, a value of -1 is returned. The
following errno error conditions are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

ENOSYS

The system doesn’t support job control.

ENOTTY

The terminal file associated with the filedes argument isn’t the
controlling terminal of the calling process.

This function is used to set a terminal’s foreground process group ID.
The argument filedes is a descriptor which specifies the terminal;
pgid specifies the process group. The calling process must be a
member of the same session as pgid and must have the same
controlling terminal.

For terminal access purposes, this function is treated as output. If it
is called from a background process on its controlling terminal,
normally all processes in the process group are sent a SIGTTOU
signal. The exception is if the calling process itself is ignoring or
blocking SIGTTOU signals, in which case the operation is
performed and no signal is sent.

If successful, tcsetpgrp returns 0. A return value of
-1 indicates an error. The following errno error
conditions are defined for this function:

EBADF

The filedes argument is not a valid file descriptor.

EINVAL

The pgid argument is not valid.

ENOSYS

The system doesn’t support job control.

ENOTTY

The filedes isn’t the controlling terminal of the calling process.

EPERM

The pgid isn’t a process group in the same session as the calling
process.

This function is used to obtain the process group ID of the session
for which the terminal specified by fildes is the controlling terminal.
If the call is successful the group ID is returned. Otherwise the
return value is (pid_t) -1 and the global variable errno
is set to the following value:

EBADF

The filedes argument is not a valid file descriptor.

ENOTTY

The calling process does not have a controlling terminal, or the file
is not the controlling terminal.